Long noncoding RNAs in cancer‐immunity cycle

Abstract

The imbalance of immune status in cancer microenvironment plays an important role in the development and progression of cancer. Immunotherapy based on this has become an important field of cancer research in recent years. Many studies on long noncoding RNA (lncRNA) in cancer have focus on its regulation in cancer development and metastasis. Recent studies have suggested that lncRNAs play crucial roles in different phases of cancer immunity, including antigen releasing, antigen presentation, immune activation, immune cells migration, infiltrating into cancer tissues, and killing cancer cells. The functional studies of lncRNAs in cancer immuntity revealed the complicated molecular mechanisms in cancer immunity from a new point of view, which may provide novel potential targets for cancer immunotherapies. Based on the classical cancer‐immunity cycle theory, we review the recent studies on the functions and mechanisms of immune‐related lncRNAs in different stages of cancer immunity, to summarize the relationship between lncRNAs, and cancer immunity and to provide a framework for further research.

The functional studies of lncRNAs in cancer immuntity revealed the complicated molecular mechanisms in cancer immunity from a new point of view, which may provide novel potential targets for cancer immunotherapies. Based on the classical cancer‐immunity cycle theory, we review therecent studies on the functions and mechanisms of immune‐related lncRNAs in different stages of cancer immunity, to summarize the relationship between lncRNAs, and cancer immunity and to provide a framework for further research.

1. INTRODUCTION

Long noncoding RNAs (lncRNAs) are a group of endogenous cellular RNAs of more than 200 bp long that encoded by mammalian genome, which couldnot encode proteins for the lack of an open reading frame (Li, Tian, Yang, & Gong, 2016). LncRNAs may function as signals, guides, decoys, and scaffolds in diverse vital physiological and pathological processes (Bonasio & Shiekhattar, 2014; Mathieu, Belhocine, Dao, Puthier, & Spicuglia, 2014; Qi, Song, & Li, 2013; Ulitsky & Bartel, 2013). In particular, studies on cancer have proved that lncRNAs actively participated in the regulaton of cancer development and progression, such as cancer growth, metastasis, and recurrence (e.g., H19 (Liang et al., 2015), HOTAIR (Zhang et al., 2014), MALAT‐1 (Li, Zou, Xie, et al., 2016). Aberrantly expressed lncRNAs in cancer can be used as biomarkers for diagnosis, prognostic prediction, and potential targets for cancer therapeutics (e.g., Xist (Tantai, Hu, Yang, & Geng, 2015).

Dysfunctional immune state in cancer microenvironment is a hallmark of cancer. During the occurrence and progression, immune system plays a dynamic cancer immunoediting role, including elimination phase, equilibrium phase, and escape phase (Teng, Galon, Fridman, & Smyth, 2015). In 2013, Mellman and Chen (2013) proposed the cancer‐immunity cycle theory, including antigen releasing, antigen presentation, immune activation, immune cells migration, infiltrating into cancer tissues, and killing cancer cells (Figure 1 ). The cancer‐immunity cycle has provided a new theoretical foundation for clinical cancer immunotherapy. On the one hand it improves cancer immunity simultaneously from cancer cells and immune cells; on the other the pertinence of cancer immunotherapy is also enhanced.

Figure 1.

LncRNAs in different steps of the cancer‐immunity cycle. Each step of the cancer‐immunity cycle can be regulated by lncRNAs, which induce or inhibit the process at an mRMA level. One kind of lncRNAs can participate in adjacent steps, such as lnc‐DC, can promote both antigen presentation, and CD4 + T cells differentiation. Examples of lncRNAs in different immune steps are shown above. VLDLR, very low density lipoprotein‐receptor; THRIL, TNF‐α, and hnRNPL‐related immunoregulatory lncRNA; CECR7, cat eye syndrome chromosome region, candidate 7; SGK1: serum and glucocorticoid‐inducible kinase 1; Th2‐LCR: Th2 locus control region; AFAP1: actin filament‐associated protein 1

Recently, mounting studies have found the crital roles of lncRNAs in cancer immunity (Figure 1 ), like NeST induce synthesis of IFN‐γ in CD8 + T cells (Gomez et al., 2013), lincRNA‐Cox2 regulate the transcription of immune genes (Carpenter et al., 2013; Hu et al., 2016; Tong et al., 2016), and GATA3‐AS1 participate in differentiation of Th2 cells (Zhang et al., 2013). However, these studies seem to be somewhat scattered without demonstrating the importance of lncRNAs in cancer immunity systematically. Therefore, we review the role of lncRNAs at various stages cancer‐immunity cycle, systematically describe their importance in cancer immunity to provide a framework for further research and clinical therapy.

Under the pressure of cancer microenvironment, immune system would be gradually reshaped, and divert from anti‐cancer to pro‐cancer ones (Alatrash, Jakher, Stafford, & Mittendorf, 2013; Geynisman, Chien, Smieliauskas, Shen, & Shih, 2014; Yang, 2015). Therefore, therapeutics focusing on re‐establishment of anti‐cancer immunity have become one of the most promising ways to restrain and eliminate cancer. Currently, the success of immune checkpoint inhibitors in clinical treatment of several cancers, like melanoma (Mahoney, Freeman, & McDermott, 2015; Ott, Hodi, & Robert, 2013), lymphoma (Jelinek, Mihalyova, Kascak, Duras, & Hajek, 2017; Sugita et al., 2017) and advanced lung cancer (Dasanu, Lippman, & Plaxe, 2017; Liu & Cho, 2017; Zhou, Liu, & Wang, 2017), forecast the hopeful prospect of immunotherapy in cancer (Brustugun, Sprauten, & Helland, 2017; Hodi et al., 2010; van der Kooij et al., 2017).

2. LncRNAs AND CANCER‐IMMUNITY CYCLE

2.1. Cancer‐immunity cycle

The immune response can be durable and adaptive, and has the potential to be self‐propagating after activation (Hamid et al., 2013). However, limited by the complexity of the immune system, the understanding of cancer immune response is not as good as the understanding of cancer itself. Cancer‐immunity cycle initiates the immune response to cancer and the self‐propagation, using the characteristics of immune response to avoid chemo‐resistance, cancer metastasis, or recurrence in late therapy. In addition, elements which can promote or inhibit anticancer immunity are summed up in the cycle. Based on this, a new frame named “cancer‐immune set point” has been put forward for further studies in both clinical therapies and fundamental researches (Chen & Mellman, 2017). The goal of cancer‐immunity cycle is to maintain the safety and efficacy of anti‐tumor immune response, each step of cancer‐immunity cycle can be used individually, or federally for patients with biological characteristics (Chen & Mellman, 2013; Karasaki et al., 2017).

2.2. LncRNAs in antigen release

In cancer microenvironment, tumor cells provide persistent chronic antigenic stimulation which may serve as activatory or inhibitory signals for immune cells (Wang, Lu, & Sun, 2017; Yatim, Cullen, & Albert, 2017). These signals have proved to be mainly produced by cancer cells with immunogenic or necrotic death. (Ferguson, Choi, & Green, 2011). lncRNAs in cancer cells regulate differently during immunogenic cell death, especially in chemotherapy, one of the clinical therapies that can lead to immunogenic cancer cell death and release neoantigens. Even the final efficacy shows high correlation with the expression of lncRNAs.

Extracellular vesicles (EVs) can lead to chemoresistance in hepatocellular carcinoma. During chemotherapeutic stress, high expression of linc‐ROR in EVs promotes the proliferation of CD133+ cells by TGF‐β, and consequently weaken the effect of chemotherapy drugs (Takahashi, Yan, Kogure, Haga, & Patel, 2014). On the contrary, EVs‐related linc‐VLDLR can enhance the efficacy of chemotherapy by regulating intercellular communication. VLDLR (very low density lipoprotein‐receptor) enhanced the expression of ATP‐binding cassette sub‐family G member 2 (ABCG2) by linc‐VLDLR, decreased cell cycle progression, reduced cell viability, and promoted chemotherapy‐related death (Takahashi, Yan, Wood, Haga, & Patel, 2014).

Some “eat‐me” signals like Calreticulin (CALR) can be rapidly transferred to cancer cell surface and recognized by dendritic cells (DCs) at the early stage of the immune death (de Bruyn, Wiersma, Helfrich, Eggleton, & Bremer, 2015; Obeid et al., 2007). To explore the interaction between ncRNA‐RB1 and the transcriptional regulation of CALR, Musahl et al. (2015) treated ncRNA‐RB1‐depleted U2OS cell with mitoxantrone (MTX), a chemotherapeutic agent that induces cell surface translocation of CALR. They found that knockdown of ncRNA‐RB1 does not have a direct impact on surface CALR, while the translocation upon MTX treatment was markedly reduced. Moreover, ncRNA‐RB1 can also use the difference to mediate recognition of tumor cells by macrophages during immunogenic cell death. Therefore, a new therapeutic target is predicted (Musahl et al., 2015).

2.3. LncRNAs participate in antigen presentation

During antigen presentation, both immunogenic signals, and functional antigen presentation cells (APCs) are demanded for the successful activation of T cells. lncRNAs involve in the expressions of various proinflammatory cytokines, their regulations in APCs maturation are also indispensable. Researches have shown that lncRNAs can significantly enrich the expression of MHC I and enhance antigen processing, suggesting that a diagnostic method based on metabolic diseases can be used to differential diagnosis of tumor and precancerous lesion (Yang et al., 2017).

Lnc‐DC is a specific marker in the cytoplasm of DCs, the most powerful antigen presenting cells (APCs) in antigen presentation (Guermonprez et al., 2013). Knockdown of lnc‐DCs reflects the differentiation of DCs, inhibits the ability of DCs to active T cells. Lnc‐DC functions by activating the transcription factor STAT3 (signal transducer and activator of transcription 3). It binds directly to STAT3, promotes STAT3 phosphorylation on tyrosine‐705 by avoiding STAT3 from being dephosphorylated by SHP1. lnc‐DC allows STAT3 to move into the nucleus constantly and maintains the transcriptional activity of STAT3, confirming the regulating effect of lncRNAs in antigen presentation (Mbongue, Nicholas, Firek, & Langridge, 2014; Wang et al., 2014).

Proinflammatory cytokines, such as IFN‐ γ and TNF‐ α, have been proved to be indispensable regulators of antigen presentation (Basler, Kirk, & Groettrup, 2013; Lippitz, 2013; Mellman, Coukos, & Dranoff, 2011; Zhou, 2009). Collier et al. (2014) showed that the expression of IFGN‐AS1 (Ifng antisense RNA 1) and IFN‐γ are high in Th1 cells, and Th1 can synthesize IFN‐γ. Knockdown of IFGN‐AS1 significantly decreases the expression of IFN‐γ, while silencing of IFN‐γ did not affect IFGN‐AS1. Such hints that the expression of IFN‐γ is regulated by IFGN‐AS1. Over expression of IFGN‐AS1 can also active the transcription of IFN‐γ (Collier, Henderson, Tossberg, & Aune, 2014). TNF‐ α induces the maturation of DC cells, and its formation is closely related to THRIL (TNF‐α and hnRNPL‐related immunoregulatory lncRNA), a kind of lncRNAs expresses in the macrophage‐like cells. A feedback loop has been proved between THRIL and TNF‐α, the silencing of THRIL decreases TNF‐α expression, while the high expression of TNF‐α leads to downregulating of THRIL (Li et al., 2014).

High mobility group box‐1 protein (HMGB1), especially those released from necrotic cells induced by chemotherapy or radiotherapy, can act as a proinflammatory factor to induce DCs maturation in antigen presentation (Chiba et al., 2012; Penza, Moniuszko, & Zajkowska, 2012; Zhu et al., 2017). Li et al. (2017) showed the connection between HMGB1 and lncRNAs in hepatocellular carcinoma. In their research, both lncRNA TP73‐AS1 and HMGB1 can combine with the same locus of miR‐200a. The interaction between TP73‐AS1 and miR‐200a weakens the inhibitory effect of miR‐200a to HMGB1, leading to the high expression of HMGB1 (Li et al., 2017).

2.4. LncRNAs regulate immune cell differentiation during immune priming and activation

The balance between T effector cells and T regulatory cells determines the final outcome of immune response (Park & Pan, 2015). A large number of lncRNAs have been checked in each phase of lymphocyte differentiation and activation, indicating that lncRNA plays an important role in this process (Kanduri et al., 2015; Pang et al., 2009; Panzeri, Rossetti, Abrignani, & Pagani, 2015; Ranzani et al., 2017; Xia et al., 2014).

The differentiation of lymphocytes needs regulatory genes to cooperate with each other as a fine system. Research from Yao et al. showed a “CECR7‐miR‐429‐CTLA4” network in diabetic pancreatic cancer (PaC). LncRNA CECR7 (cat eye syndrome chromosome region, candidate 7) regulates the expression of cytotoxic T‐lymphocyte‐associated protein 4 (CTLA4) by targeting miR‐429 (Yao, Wang, Jia, & Zhao, 2017). Downregulation of CECR7 increased the expression of CTLA4 in cell surface. CECR7 might be a therapeutic target in diabetic PaC. Another example is about gastric cancer. linc‐POU3F3 promotes the distribution of Tregs in peripheral blood T cells (Xiong, Yang, Chen, & Fan, 2015). In researches of gastric cancer caused by Helicobacter pylori infection or highsalt diet, lnc‐SGK1 (lnc‐serum and glucocorticoid‐inducible kinase 1) promotes Th2 and Th17 differentiation through SGK1/JunB signaling (Yao et al., 2016).

Linc‐MAF‐4 is a marker lncRNA located in the upstream of MAF gene in Th1 cells. MAF is a Th2 cell related transcription factor involved in the differentiation of Th2 cells. According to the findings by Ranzani and his colleagues, Linc‐MAF‐4 acts as a scaffold to recruit repressor protein enhancer of zeste homolog 2 (EZH2) and lysine specific demethylase 1 (LSD1), which acts on the promoter region of MAF and inhibits the transcription of MAF, thereby skewing the differentiation of T cells toward the Th2 phenotype (Ranzani et al., 2015; Zhang, Liu, Wei, Gao, & Hao, 2017). Th2‐LCR (Th2 locus control region) is another lncRNA that involved in Th2 cell expression. It promotes the formation of H3K4Me3 marks at multiple genomic positions of IL‐4 and IL‐13 by lncRNA alternative splicing, promoting the expression of Th2 cells (Hwang et al., 2013; Spurlock et al., 2015).

Since antigen presentation is closely related to activation of the immune system, lncRNAs involved in antigen presentation is also involved in the regulation of immune cell differentiation. For example, recent research by Zhang et al. has found that lnc‐DC can be overexpressed through the p‐STAT3 pathway in preeclampsia patients. This induces the over‐maturation of decidual DCs so that CD4 + T cells could be induced to differentiate into Th1 cells and finally lead to an increase in inflammation (Zhang, Zhou, & Ding, 2017).

2.5. LncRNAs influence immune cells migration and t cells infiltration

Recently, several articles have provided mounting evidences that lncRNAs participate in immune cells migration and infiltration in cancer. In Th2 cells, genes like Ccr1, Ccr2, Ccr3, and Ccr5 are required for the migration to lung tissues. Knockdown of lincR‐Ccr2‐5′AS decreases the expression of these genes and inhibits the trafficking of Th2 cells to lung tissues, suggesting the direct function of lncRNAs in the migration of T regulatory cells (Hu et al., 2013).

Indoleamine 2,3‐dioxygenase 1 (IDO1) has been reported to promote the infiltration and differentiation of Treg cells in tumor microenvironment (Bonanno et al., 2012). According to the researches by Wu et al. lnc‐sox5 acts as a tumor oncogene in colorectal cancer (CRC). Knockdown of lnc‐sox5 remarkably suppresses the mRNA level of IDO1. Since CD3 + CD8 + CTLs in CRC is reflected by Tregs, lnc‐sox5 knock‐down indirectly increase the frequency of CD3 + CD8 + CTLs, and their cytotoxicity is also dramatically enhanced. These effects can finally lead to unbalance of tumor microenvironment. lnc‐sox5 may become a therapeutic target for immune cells infiltration in CRC (Wu et al., 2017).

2.6. LncRNAs affect the recognition and killing of cancer cells

Functions of lymphocytes infiltrating in tumor microenvironment can be differently influenced by many immune mechanisms. For example, over expression of programmed death 1(PD‐1) enhance depletion and deactivation of T cells in tumor tissues (Ishida, Agata, Shibahara, & Honjo, 1992; Muenst et al., 2013; Ohashi, 2016; Pedoeem, Azoulay‐Alfaguter, Strazza, Silverman, & Mor, 2014; Wang, Schoenhals, et al., 2017). Researches on nasopharyngeal carcinoma (NPC), a kind of cancer with high level of PD‐1 (Fang et al., 2014; Zhang et al., 2015), shown that the expression of lncRNA AFAP1‐AS1 (actin filament‐associated protein1‐AS1) is positively correlated with PD‐1, and both high expression of AFAP1‐AS1 and PD‐1 predict poor prognosis in NPC. Since the specific mechanism remains unknown, we may treat AFAP1‐AS1 as a potential target for stimulating of anti‐tumor response in NPC (Tang et al., 2017).

Sehgal et al. (2014) have reported that the expression of Fas‐AS1 was negatively correlated with the expression of the soluble Fas (sFas). In non‐Hodgkin lymphoma, mutations or overexpression of EZH2 lead to hypermethylation of the promoter region of Fas‐AS1 and suppressing its expression, which could finally result in a poor prognosis. Fas‐AS1 can bind with RBM5 and inhibit the expression of sFas. In tumor immunity, via binding to Fas molecules on target cells, Fas ligands transmit apoptotic signals into the cell, then the DNA degrading enzymes are activated, which causes the apoptosis of cancer cells (Sehgal et al., 2014; Zhou, Su, & Zhou, 2016).

3. OUTLOOK

The achievement of immune checkpoint inhibitors in clinic has confirmed the feasibility and superiority of cancer immunity. However, only a fraction of patients exhibit durable responses, which hints that other mechanisms may restrict the immune response. Since both innate immunity and adaptive immunity are highly dependent on a series of regulation at the mRNA level (Fitzgerald & Caffrey, 2014), lncRNAs are now considered as an important part in cancer immunotherapy. Based on this, lncRNAs can be used to ascertain the restrictive immune responses. To expand the scope of cancer immunotherapy, those who have involved in the regulation may be knockdown to reduce expression of restricted genes, or be enhanced to strengthen the anti‐cancer immune response. Current researches mainly discuss the biology of cancer cells, while the understanding of immune response to cancer is not enough. Cancer‐immunity cycle based on adaption and self‐propagation of immune response, treats cancer cells from different steps, which enhances cancer immunotherapy pertinently. Understanding the role of lncRNAs in cancer‐immunity cycle aims to identify the mechanisms of cancer immune response, providing new therapeutic targets for cancer intervention from an mRMA level. LncRNAs, as biomarkers, tab different stages in cancer immunity, which enables immunotherapy to be adjusted and more consistent with the patient's biological characteristics.

CONFLICTS OF INTEREST

The authors declare no conflict of interest.

Yu W‐D, Wang H, He Q‐F, Xu Y, Wang X‐C. Long noncoding RNAs in cancer‐immunity cycle. J Cell Physiol. 2018;233: 6518–6523. 10.1002/jcp.26568